1. Field of the Invention
The present invention relates to high-intensity ionizers which pre-charge particulate matter entrained in a contaminated gas stream prior to removal of the charged particles from the stream by electrostatic precipitation. More specifically, the invention is directed to an improved electrode configuration for a co-axial venturi ionizer wherein jets of clean gas are injected into the venturi through a series of circumferential nozzles and flow over the ionizer anode surface in a laminar film to prevent particulate deposition thereon.
2. Description of the Prior Art
Standards for emissions of particulate in flue gases issuing from coal fired electrical power station stacks are becoming increasingly more stringent. Current air quality standards require that more than 99% of the fly ash produced by burning coal be removed prior to discharge of the combustion gases from the stack. Thus, the efficiency of particulate collection must increase in proportion to the ash content of the coal. In addition, in an effort to reduce the emission of certain gaseous pollutants, particularly the sulfur oxides, it has become increasingly necessary to use low sulfur coal in electrical power generating plants.
The electrostatic precipitator is the most commonly used device for the removal of particulate matter from power station stack gases. Because the size of an electrostatic precipitator is determined by the efficiency of fly ash removal required, an increase in required fly ash collection efficiency requires a corresponding increase in equipment size and cost. Moreover, because fly ash resistivity tends to be inversely related to the level of combustible sulfur in the coal burned, the use of low sulfur coals to directly reduce gaseous sulfur oxide emissions, produces highly resistive dusts. It has been demonstrated that the size of the electrostatic precipitator necessary to achieve a given level of collection efficiency increases with increasing electrical resistivity of the fly ash. The use of low sulfur coals therefore further increases the size and cost of the precipitator.
Recently, high-intensity ionizers have been developed in which a unique electrode geometry produces a stable high-intensity corona discharge through which the particulate-laden gas is passed. The ionized flue gases produced charge the particulate matter to a much higher level than is achievable with a conventional electrostatic precipitator. When the ionizer is followed with an electrostatic precipitator, the higher particle charge results in a higher collection efficiency in the precipitator due to higher migration or particle drift velocity. In such a two-stage arrangement, the ionizer acts as the charging stage and the precipitator serves as the collecting stage.
Such high-intensity ionizers utilize a co-axial pair of electrodes to generate a high-intensity field expanding radially and axially with respect to the direction of gas flow. The anode in such an arrangement typically takes the form of a venturi diffuser through which the stack gases flow immediately prior to entering the precipitator stage. The cathode is a disk co-axially mounted within the venturi throat and is formed with a curved peripheral edge having a radius much smaller than the inner radius of the venturi. When a high voltage power supply is connected between the anode and cathode, a high-intensity corona discharge is established in an annular region between the arcuate periphery of the cathode disk and the surrounding cylindrical anode surface near the disk. Because the field is relatively narrow in the direction of gas flow, a high intensity field is achievable without prohibitive power requirements. The combination of the high gas stream velocity through the venturi and the high intensity transverse electric field through which the gas stream passes produces intense ionization and very high levels of charge on the particles and results in increased collection efficiency notwithstanding the high resistivity of the particulate as in the case of fly ash from low sulfur coal.
One of the problems which has been encountered in connection with co-axial high intensity ionizers of the type described above results from the detrimental build-up of charged particles of the cylindrical anode wall near the corona discharge plane. Deposition of high resistivity particulate matter in this region results in the phenomena of back corona and excessive sparking with a resulting deterioration in the electrical field and degradation in particle charge. Prior attempts to overcome this problem have involved "cleaning" the anode surface in the affected region to eliminate disturbances in the corona due to contaminate build-up on the outer electrode. This cleaning has been accomplished by injecting water or similar fluid onto the surface of the converging cone section of the venturi wall. The venturi is pointed in a downward direction and the water flows over the anode surface under the combined action of gravity and friction with the moving gases.
Another approach has been to utilize a venturi with a porous or perforated anode wall or a screen through which clean gas from an external source is introduced into the venturi in a direction normal to the main gas stream to form a clean gas protective barrier. The latter approach, however, has not yielded totally satisfactory results for several reasons.
First, large amounts of clean gas are required with this approach. In addition, the sharp edges and small protuberances on the surface of screens, perforated plates or porous metal ionizer walls can serve as sites for back corona initiation. Also, screens, perforated plates, wire wound cylinders with external tie bars or porous materials all have areas of low jet velocity either at the area where sheet material is joined to form a cylinder or in the wake of external structural members. Particulate matter tends to accumulate in these areas degrading the performance by promoting local back corona.